Molten salt synthesis of barium and/or strontium titanate powder

ABSTRACT

A particulate mixture of an alkali metal carbonate solvent, titanium oxide, and an alkaline earth reactant selected from the group consisting of barium oxide, strontium oxide, and mixtures thereof, is heated to melt the alkali metal carbonate solvent in which the reactants dissolve and react precipitating a titanate selected from the group consisting of barium titanate, strontium titanate and mixtures thereof.

This application is a continuation of application Ser. No. 512,082, filed July 8, 1983, which is a continuation of Ser. No. 394,356, filed July 1, 1982, now abandoned.

The present invention relates to the preparation of barium titanate powder, strontium titanate powder, and mixtures, i.e. solutions, thereof.

Conventionally, barium or strontium titanate powder is prepared by the high temperature solid state reaction of appropriate precursor compounds. The reaction product is in the form of relatively large, strong self-bonded particle aggregates which must be comminuted to the desired approximately 1.0 micron particle size before fabrication into ceramic articles.

One of the inadequacies of this conventional procedure is that the product, in commercial practice, is not fully reacted to yield uniform stoichiometry on a microscopic level. In cases where the physicochemical properties depend on the composition and its uniformity, the nonuniform composition of the conventional product can lead to less than optimum properties. Although procedures can be adopted in the conventional process to minimize these variations, the penalty in additional effort can be considerable.

The comminution portion of the conventional process is also potentially detrimental in that significant, uncontrollable quantities of undesirable impurities can be introduced from the grinding media. Again, extraordinary precautions can be taken to minimize the comminution effects, but with attendant penalties.

The present invention circumvents the inadequacies of the conventional process by substituting for the solid state reaction, a liquid phase reaction scheme utilizing a molten carbonate solvent with subsequent precipitation of the product which does not require comminution. In the present process, the reactants are slightly soluble in the molten carbonate solvent, therefore allowing literally atomic mixing in the liquid phase of the reactants. The solubilities of the reactants are such that they exceed the corresponding solubilities determined by the solubility product of the product in the molten carbonate solvent. Hence, the reaction product precipitates spontaneously from the molten carbonate solution. The reactants will continuously dissolve to maintain a reactant-saturated solution until they are totally consumed by product formation.

The following copending U.S. patents are assigned to the assignee hereof and are incorporated herein by reference:

U.S. Pat. No. 4,233,282 to R. H. Arendt for "Molten Salt Synthesis of Barium and/or Strontium Titanate Powder" discloses a process consisting essentially of forming a mixture of an alkali metal chloride solvent salt, titanium oxide and an alkaline earth reactant selected from the group consisting of barium oxide, strontium oxide, and mixtures thereof, and heating said mixture to melt the chloride salt solvent in which the titanium oxide and alkaline earth reactant dissolve and react precipitating a titanate selected from the group consisting of barium titanate, strontium titanate and mixtures thereof.

U.S. Pat. No. 4,293,534 to R. H. Arendt for "Molten Salt Synthesis of Alkaline Earth Titanates, Zirconates and their Solid Solutions" discloses a process consisting essentially of forming a mixture of an alkali metal hydroxide solvent salt, a reactant selected from the group consisting of titanium oxide, zirconium oxide and mixtures thereof, and an alkaline earth reactant selected from the group consisting of barium oxide, strontium oxide, and mixtures thereof, and heating the resulting mixture to melt the alkali metal hydroxide solvent in which the reactants dissolve and react precipitating a titanate, zirconate or solid solutions thereof.

Briefly stated, the present process for producing a powder of an alkaline earth titanate wherein the alkaline earth component is selected from the group consisting of barium, strontium and mixtures thereof, consists essentially of providing particulate TiO₂ in at least stoichiometric amount or particulate precursor therefor, providing particulate alkaline earth oxide selected from the group consisting of BaO, SrO, and mixtures thereof in at least stoichiometric amount or particulate precursor therefor, providing an alkali metal carbonate selected from the group consisting of sodium carbonate, potassium carbonate, lithium carbonate and mixtures thereof, forming a mixture of said TiO₂ or precursor therefor, said alkaline earth oxide or precursor therefor and said carbonate solvent, said carbonate solvent being used in an amount of at least about 20% by weight of the total amount of said TiO₂, said alkaline earth oxide and said carbonate solvent, heating said mixture to a reaction temperature at least sufficient to melt said carbonate solvent and ranging from about 873 K. to about 1473 K., each said precursor decomposing completely at or below said reaction temperature to form said oxide and by-product gas, said carbonate solvent in molten form being a solvent for said TiO₂ and alkaline earth oxide, maintaining said reaction temperature dissolving and reacting said TiO₂ and alkaline earth oxide in said molten solvent and precipitating said alkaline earth titanate, and recovering said precipitated alkaline earth titanate.

The reactions for producing the present alkaline earth products are as follows: ##STR1##

All of the reactants are used in at least stoichiometric amounts to achieve complete reaction. Preferably, as a practical matter and to insure production of a completely reacted product, BaO and/or SrO are used in an amount ranging from about 2% by weight to about 10% by weight in excess of stoichiometric. There is no advantage in using amounts of BaO and/or SrO higher than 10% by weight in excess of stoichiometric.

The present process produces barium and/or strontium titanate. Since the crystals of barium and/or strontium titanate are produced by precipitation from a saturated solution, they are chemically homogeneous, i.e. they are of uniform composition on a microscopic level. The average crystal size of the individual crystals is determined largely by reaction temperature and carbonate solvent content and ranges from about 0.3 micron to about 2 microns. The higher the reaction temperature and/or the greater the amount of carbonate, the larger is the average crystal size of the resulting titanate product. The crystals of the present product are or would be expected to be roughly cubic.

In carrying out the present process, the reactants or precursors therefor can be of commercial or technical grade, and their particular purity depends largely on the particular application of the resulting alkaline earth titanate. Specifically, the reactant oxides should not contain any impurities which would have a significantly deleterious effect on the resulting alkaline earth titanate powder or on its particular application.

The reactant oxides or precursors therefor should be of a size which allows the reaction to take place. Generally, these reactants or precursors therefor are used and are satisfactory in the particle size range in which they are available commercially, which ordinarily ranges from submicron up to about 100 microns. The reactant powders should also be free of large, hard aggregates, i.e. significantly above 100 microns in size, which might survive the mixing process and prevent sufficient reactant contact for satisfactory reaction rates.

In the present process the carbonate solvent is selected from the group consisting of sodium carbonate, potassium carbonate, lithium carbonate and mixtures thereof. The carbonate solvent is used in a minimum amount of at least about 20% by weight of the total amount of reactant oxides and carbonate solvent. Lithium carbonate as a solvent component is an exception in that when it is used it will also produce a small amount of Li₂ TiO₃, but the formation of Li₂ TiO₃ can be suppressed by using the alkaline earth oxide reactant in an excess amount of at least 10 weight %. Typically, the present carbonate solvent is used in an amount of about 50% by weight of the total amount of reactants and carbonate solvent. Amounts of carbonate solvent lower than about 20% by weight are not practical since the desired reaction will not proceed to completion. There is no upper critical maximum amount of carbonate, but amounts of carbonate higher than about 70% by weight of the total amount of reactant oxides and carbonate solvent provide no significant advantage. As the amount of carbonate is increased from 20% by weight to about 50% by weight, the amount of individual grains or crystals present in the resulting alkaline earth titanate powder increases correspondingly as does the individual crystal size, and with an amount of carbonate solvent above about 50% by weight, substantially all or all of the titanate powder recovered consists essentially of individual grains or crystallites.

The reactants or precursors therefor and the alkali carbonate solvent are admixed to produce a substantially thorough mixture to insure good contact and complete reaction. The actual mixing operation can be carried out in several ways which do not introduce undesirable impurities into the resulting product. Preferably, water at room or ambient temperature is admixed with the reactant oxides or precursors therefor and the carbonate solvent in an amount which, with stirring, is at least sufficient to form a slurry. The wet mixing can be carried out, for example, using a plastic milling medium or by wet mixing in a high speed blender with preferably distilled or deionized water, depending on the application of the resulting product and preferably, with stainless steel or plastic stirrers, for example, a propeller, in a stainless steel or plastic, preferably Teflon, lined vessel.

The present reaction temperature ranges from about 873 K. (600° C.) to about 1473 K. (1200° C.). A reaction temperature below about 873 K. produces a product which is incompletely reacted, i.e. it contains unreacted TiO₂. On the other hand, a temperature higher than about 1473 K. is likely to cause significant vapor loss of the molten carbonate solvent and crucible interaction. The particular reaction temperature used depends largely on the reaction rate desired, i.e. rate of precipitation of alkaline earth titanate desired, and crystal size desired. In the present process, the higher the reaction temperature, the faster is the reaction rate and the larger are the resulting product crystals. Preferably, the reaction temperature ranges from about 1073 K. to about 1273 K. since it is not difficult to maintain and provides high reaction rates without significant vapor loss of the molten carbonate solvent.

In molten form, the present carbonate is a solvent for the reactant oxides. For sodium carbonate alone the melting point, i.e. liquidus temperature, is about 1131 K., whereas for potassium carbonate alone it is about 1171 K. All mixtures of potassium carbonate and sodium carbonate and/or lithium carbonate melt below the melting point of potassium carbonate alone. As shown by their phase diagrams, for the eutectic mixture of 58 mole % sodium carbonate/balance potassium carbonate, the liquidus temperature is about 983 K. For the eutectic 53.3 mole % Li₂ CO₃ /balance Na₂ CO₃, the liquidus temperature is about 769 K. For the eutectic 42.7 mole % Li₂ CO₃ /balance K₂ CO₃, the liquidus temperature is about 771 K., whereas for the eutectic 62 mole % Li₂ CO₃ /balance K₂ CO₃, the liquidus temperature is about 761 K.

Preferably, in the present invention, the minimum reaction temperature is at least about 10 K. above the liquidus temperature of the carbonate solvent since such a temperature insures complete melting of carbonate solvent and also increases the fluidity of the molten carbonate producing increased wetting of the reaction mixture. Reaction temperatures always should be maintained below the melting point of the alkaline earth product being precipitated, which for SrTiO₃ is about 2311 K. and for BaTiO₃ is about 1889 K.

At reaction temperature the carbonate is molten and the reactant oxides dissolve and react in the molten carbonate precipitating the alkaline earth titanate. Reaction temperature is maintained until the reaction is completed. Reaction time is a strong function of the reaction temperature used, varying in an inverse fashion. For reaction temperatures ranging from about 1073 K. to about 1273 K., a reaction time of about two hours for complete reaction is suitable. When the reaction is completed, the reacted mass is cooled at any convenient rate, preferably in air and preferably to about room temperature.

The cooled alkaline earth titanate-containing reacted mass is a solid, fired cake comprised of alkaline earth titanate particles or crystals distributed throughout a matrix of solidified carbonate. Specifically, the alkaline earth titanate, which is maintained as distinct second phase crystals, is present as fine crystals embedded in the carbonate matrix.

The present alkaline earth titanate can be recovered by a number of techniques which dissolve away the carbonate solvent. Preferably, the titanate-containing reacted mass is contacted and admixed at room temperature with dilute aqueous acid, preferably having a pH of about 4 to 5, preferably acetic acid, in an amount sufficient to react quantitatively with any excess barium or strontium component to produce a compound which is soluble in the resulting acidic solution. For example, acetic acid produces the acetate of barium and/or strontium which is soluble in aqueous acetic acid. The acid also preferably converts the carbonate of potassium, sodium or lithium to a compound which also is soluble in the resulting aqueous acidic solution leaving the present titanate powder dispersed in the resulting aqueous solution. For example, the acetic acid also converts the carbonate of potassium, sodium or lithium to a compound which also is soluble in the aqueous acetic solution leaving the present titanate powder dispersed in an aqueous solution of acetates. A flocculating agent which is organic and soluble in water, preferably an anionic flocculant, is added in an amount which is sufficient to flocculate and settle the alkaline earth titanate powder. Preferably, the flocculating agent is added in aqueous solution, about 0.1 gram of flocculant per 100 grams of water usually being satisfactory, and preferably the flocculating agent is used only in an amount necessary to settle the powder. The supernatant liquid is then decanted.

Preferably, after several washings with water and settlings with flocculating agent and decantations of the supernatant liquid, the wet powder is dried. For production of barium and/or strontium titanate powder of high purity, the flocculating agent should be completely removable by heating the powder in air at temperatures below the melting point of the powder and preferably not higher than about 1073 K. Preferably, to remove any remaining water and flocculant, it is heated at about 523 K. for about 20 minutes.

Alternatively, in the present process, if desired, a particulate inorganic precursor of the reactant oxides can be used. The precursor should decompose completely to form the oxide and by-product gas or gases leaving no contaminants in the reacted mass. Representatives of the precursors of the reactant oxides useful in the present process are the carbonates, hydroxides and nitrates. The precursor should be used in an amount sufficient to produce the respective oxide in at least stoichiometric amount.

The present alkaline earth titanate powder ranges in color from a slightly off-white to a tan. The alkaline earth titanate powder is free-flowing and can be in the form of aggregates or in the form of individual powder crystals, but usually it is a mixture of both. The aggregates which range in size from about 1 micron to about 20 microns, are particles consisting essentially of a cluster of smaller sized crystals weakly bonded together autogeneously, i.e. such bonding is believed to be caused by Van der Waal's forces or by self-bonding, i.e. neck growth between grains. The aggregates are friable at room temperature and are readily broken down with very gentle comminution during subsequent pressing into a green body. The bigger the crystal, the less the aggregation. Thus, the present crystals approaching an average crystal size of about 2 microns are monodispersed.

The present alkaline earth titanate powder can be prepared free of impurities or free of significant impurities. Therefore, when the same procedure is used, its properties are reproducible from batch to batch.

The present alkaline earth titanate powder has a number of uses. It has good dielectric properties and it could be admixed with, for example, a polymer to produce a composite with improved dielectric properties. Alternatively, it can be pressed at room temperature into a green body of desired size and shape. For example, the SrTiO₃ powder in such pressed form would be useful as an electrolyte supporting matrix in a molten carbonate fuel cell. Alternatively, the green body could be fired at temperatures below its melting point in an oxygen-containing atmosphere such as air to produce fired polycrystalline bodies with densities of at least about 85% of theoretical density. Alternatively, the titanate powder could be hot-pressed in an oxide die, preferably an alumina die press at temperatures of about 1573 K. to 1673 K. under a pressure of at least about 40 MPa to produce a hot pressed product having a density higher than 95% of theoretical density. These sintered or hot pressed products are useful, for example, as dielectric materials for capacitors and electrical circuits.

The invention is further illustrated by the following examples:

EXAMPLE 1

In this example, strontium titanate (SrTiO₃) was prepared.

556.75 grams of reagent grade SrCO₃ (2% in excess of stoichiometry for SrTiO₃) ranging in size up to about 100 microns and 295.41 grams of reagent grade TiO₂ ranging in size up to about 100 microns were slurried at room temperature in absolute methanol (˜1600 ml) for 20 minutes, then set to dry overnight in an infrared-heated oven. The resulting dried material was lightly comminuted in a mortar and pestle, then placed in a gallon glass jar with 245.40 g of 99% pure Li₂ CO₃ and 279.08 g of 99.8% pure K₂ CO₃ (dried at 523 K. for several hours and comminuted). The total powder mixture was homogenized by rolltumbling for 10 minutes.

Equivalent amounts of the mixture were placed in three dense, covered 500 ml α-Al₂ O₃ crucibles and heated at 50 K. hour ⁻¹ to 928 K. and held at 928 K. for 24 hours in an air atmosphere, then furnace cooled to room temperature.

A sample of the resulting lightly sintered, compacted mass was treated with aqueous 10% CH₃ COOH to dissolve away the alkali carbonates and excess Sr⁺² salt. The freed SrTiO₃ crystals were collected on a filter, washed with water, and dried.

The product was an off-white powder. X-ray diffraction analysis of the product indicated the product to be phase pure SrTiO₃. B.E.T. surface area measurements of the product yielded a value of 1.24 meters² gram⁻¹ which indicates a spherical equivalent average crystal diameter (SED) of 0.947 micron.

About 2.0 grams of the SrTiO₃ powder were compressed in a stainless steel die at room temperature under a pressure of 13.7 MPa to produce a compact.

Mercury intrusion porosimetry measurement of the resulting compact showed a total pore volume fraction of 0.5012, with a mean pore size of 0.611 μm.

X-ray diffraction analysis showed the product to be phase pure SrTiO₃ (the small Li₂ TiO₃ content was not detected).

EXAMPLE 2

A "master" SrCO₃ -TiO₂ reactant mix containing 1230.79 g SrCO₃, 653.06 g TiO₂ was prepared as disclosed in Example 1, except that H₂ O replaced the methanol and drying was carried out at 423 K. in a convection oven. A "master" solvent mix of Na₂ CO₃ -40 mol % K₂ CO₃ containing 802.43 g Na₂ CO₃ and 697.57 g K₂ CO₃ (dried as in Example 1) was roll-tumbled for 10 minutes in a gallon glass jar to homogenize. Three separate reaction mixtures were prepared, each based on 625.00 g of the master reactant mix and 25 (165.89 g), 37.5 (298.59 g) and 50 wt% (494.70 g) of solvent mix respectively. These were, homogenized by roll-tumbling for 10 minutes.

Each mixture was divided into three approximately equal aliquots, each contained in an individual dense, covered 500 ml α-Al₂ O₃ crucible. A group of three aliquots, one with each of the solvent contents, three groups in all, was reacted by heating to the reaction temperature (1073, 1173 and 1273 K.) at 100 K. hour⁻¹ in an air atmosphere, holding 2 hours at reaction temperature, then cooling at <150 K. hour⁻¹ to room temperature. A total of nine samples, covering three solvent contents and three reaction temperatures were thus generated.

Each sample was placed in ˜5 liter distilled H₂ O and, with vigorous stirring, sufficient concentrated CH₃ COOH added dropwise to convert the alkali carbonates and excess Sr⁺² to the respective acetates which dissolved away. The freed SrTiO₃ powder was collected, processed and evaluated as disclosed in Example 1. Compacts of the SrTiO₃ powder were produced and evaluated in the same manner disclosed in Example 1.

These runs are illustrated in Table I.

                  TABLE I                                                          ______________________________________                                                               Compact                                                                        of SrTiO.sub.3                                                                 Product Powder                                                   SrTiO.sub.3 Product Powder                                                                     Total    Mean                                                        Carbonate BET         Porosity                                                                              Pore                                     Reaction Solvent   Area   SED  (Volume                                                                               Size                                Run  Temp.    Content   (m.sup.2 g.sup.-1)                                                                    (μm)                                                                             Fraction)                                                                             (μm)                             ______________________________________                                         2A   1073 K.  25 wt %   4.27   0.275                                                                               0.6168 0.328                               2B   1073 K.  37.5 wt % 3.36   0.349                                                                               0.5636 0.259                               2C   1073 K.  50 wt %   2.82   0.416                                                                               0.5554 0.255                               2D   1173 K.  25 wt %   2.36   0.498                                                                               0.5247 0.338                               2E   1173 K.  37.5 wt % 1.63   0.720                                                                               0.5401 0.417                               2F   1173 K.  50 wt %   1.40   0.839                                                                               0.5552 0.527                               2G   1273 K.  25 wt %   0.99   1.186                                                                               0.5259 0.569                               2H   1273 K.  37.5 wt % 0.86   1.365                                                                               0.4999 0.688                               2I   1273 K.  50 wt %   0.61   1.925                                                                               0.5093 0.990                               ______________________________________                                    

X-ray diffraction analysis of the SrTiO₃ powder produced in all Runs indicated the powder was phase pure SrTiO₃.

All of the Runs of Table I illustrate the present invention. Table I shows that by increasing the reaction temperature, the average crystal size of the SrTiO₃ product powder is increased significantly. Table I also shows that the greater the carbonate solvent content, the larger is the average crystal size of the resulting SrTiO₃ powder.

The effect of crystal size on compact interstitial void volume is graphically demonstrated, i.e. the pore volume increased significantly as the crystal size was reduced.

The compacts of Runs 2A to 2I would be useful as matrices in a molten carbonate fuel cell.

EXAMPLE 3

Past experiments have indicated that BaO reacts in substantially the same manner as SrO, and in that sense, it is substantially a chemical equivalent of SrO.

This is a paper example for the preparation of BaTiO₃. The procedure of this example is the same as that disclosed in Example 1 except that BaCO₃ is substituted for SrCO₃, and the resulting BaTiO₃ will not differ significantly in size from the SrTiO₃ powder produced in Example 1.

Ser. No. 394,357 for Preparation of Large Crystal Sized Barium and/or Strontium Titanate Powder filed on July 1, 1982, now abandoned in favor of Ser. No. 522,085 filed on Aug. 11, 1983 in the name of R. H. Arendt, assigned to the assignee hereof and incorporated herein by reference discloses that a mixture of an alkali carbonate solvent salt, titanium oxide, an alkaline earth oxide selected from the group consisting of barium oxide, strontium oxide, and mixtures thereof and alkaline earth titanate seed is heated to melt the carbonate salt solvent in which the titanium oxide and alkaline earth oxide dissolve and react precipitating a titanate selected from the group consisting of barium titanate, strontium titanate and mixtures or solutions thereof onto said titanate seed producing large crystal sized barium and/or strontium titanate powder, said titanate seed not differing significantly in chemical composition from the titanate being precipitated thereon. 

What is claimed is:
 1. A process for producing alkaline earth titanate powder having an average crystal size ranging from about 0.3 micron to about 2 microns which consists essentially of providing particulate TiO₂ in stoichiometric amount, providing particulate alkaline earth carbonate in an amount ranging from about 2% by weight to about 10% by weight in excess of stoichiometric amount, said alkaline earth carbonate being selected from the group consisting of BaCO₃, SrCO₃ and mixtures thereof, providing an alkali carbonate salt selected from the group consisting of sodium carbonate, potassium carbonate, lithium carbonate and mixtures thereof, forming a mixture of said TiO₂, said alkaline earth carbonate and said carbonate salt, said alkali carbonate salt being used in an amount from about 20% by weight to about 70% by weight of the total amount of said TiO₂, said alkaline earth carbonate and said carbonate salt, heating said mixture to a reaction temperature at least sufficient to melt said carbonate salt and ranging from about 873 K. to about 1473 K., each said alkaline earth carbonate being decomposible at said reaction temperature to form the oxide and by-product gas, said carbonate salt in molten form being a solvent for said TiO₂ and alkaline earth carbonate, maintaining said reaction temperature, continually dissolving and reacting said added TiO₂ and alkaline earth carbonate in said molten salt producing by-product gas and precipitating said alkaline earth titanate at said reaction temperature, at said reaction temperature said TiO₂ and said alkaline earth carbonate being slightly soluble in said molten salt to maintain a reactant-saturated solution, said average crystal size of said alkaline earth titanate powder increasing with increasing reaction temperature and/or with increasing amount of said molten salt, and recovering said precipitated alkaline earth titanate.
 2. A process for producing strontium titanate powder having an average crystal size ranging from about 0.3 micron to about 2 microns which consists essentially of providing particulate TiO₂ in stoichiometric amount, providing particulate SrCO₃ in an amount ranging from about 2% by weight to about 10% by weight in excess of stoichiometric amount, providing an alkali carbonate salt selected from the group consisting of sodium carbonate, potassium carbonate, lithium carbonate and mixtures thereof, forming a mixture of said TiO₂, said SrCO₃ and said carbonate salt, said carbonate salt being used in an amount of from about 20% by weight to about 70% by weight of the total amount of said TiO₂, SrCO₃ and said carbonate salt, heating said mixture to a reaction temperature at least sufficient to melt said carbonate salt and ranging from about 873 K. to about 1473 K., said SrCO₃ being decomposible at said reaction temperature to form the oxide and by-product gas, said carbonate salt in molten form being a solvent for said TiO₂ and SrCO₃, maintaining said reaction temperature, continually dissolving and reacting said added TiO₂ and SrCO₃ in said molten salt and precipitating said strontium titanate at said reaction temperature, at said reaction temperature said TiO₂ and said SrCO₃ being slightly soluble in said molten salt to maintain a reactant-saturated solution, said average crystal size of said strontium titanate powder increasing with increasing reaction temperature and/or with increasing amount of said molten salt, and recovering said precipitated strontium titanate.
 3. The process according to claim 2 wherein said reaction temperature ranges from about 1073 K. to about 1273 K. and is at least about ten degrees higher than the liquidus temperature for said carbonate solvent.
 4. The process according to claim 2 wherein said strontium titanate is recovered by dissolving away said alkali carbonate. 